1. Field of the Invention
The present invention relates to a valve-driving system for driving an intake valve or an exhaust valve of an internal combustion engine.
2. Description of the Related Art
An intake valve or an exhaust valve of a conventional internal combustion engine is opened and closed by power taken out from a crank shaft of an internal combustion engine. In recent years, however, an attempt has been made to drive the intake valve or the exhaust valve by means of an electric motor. For example, Japanese Patent Application Laid-open No. 8-177536 discloses a valve-driving apparatus which drives a cam shaft by a motor to open and close the intake valve for driving an EGR valve, and there is also known a valve-driving apparatus which converts rotation of a motor into a straight opening and closing motion of the valve utilizing a screw mechanism provided on a valve stem (see JP-A No. 10-73178).
Since the apparatus which converts rotation of a motor into opening and closing motion of a valve by means of the screw mechanism is such that a necessary amount of rotation of the motor is great, thus being inefficient, it is not suitable as a driving apparatus of an intake valve or an exhaust valve which requires to operate the valve at high speed and periodically.
On the other hand, when the cam shaft is rotated by a motor, it is possible to drive the intake valve or the exhaust valve efficiently. In a multi-cylinder internal combustion engine which is generally used as a power source of a vehicle, a cam shaft is commonly used between a plurality of cylinders arranged in a single line. If the commonly used cam shaft is only driven by the motor, the variation of motion of the cam shaft affects operation characteristics of all of the intake valves and exhaust valves which are driven by the cam shaft. Therefore, flexibility of operation characteristics which are obtained by controlling the motor is not so high.